Early Studies Suggest It May Damage Vulnerable Cancer Cells, Reduce Treatment Side Effects, and Possibly Improve Outcomes for Some Patients

Linus Pauling was widely mocked for claiming that vitamin C could fight cancer, especially after trials using vitamin C pills showed no benefit. Researchers now know that intravenous vitamin C reaches vastly higher concentrations and can behave more like an experimental cancer drug than an ordinary supplement. Early studies suggest it may damage vulnerable cancer cells, reduce treatment side effects, and possibly improve outcomes for some patients.

Linus Pauling was one of the most brilliant scientists of the 20th century. He won two Nobel prizes and transformed our understanding of chemical bonds and the structure of proteins. Late in his career, though, he became famous for something very different: a passionate belief that very high doses of vitamin C could help people with cancer.

Many doctors scoffed. When Pauling himself later died of cancer aged 93, he was held up as a classic case of the "halo effect": being a genius in one field doesn't guarantee wisdom in another. Half a century on, the story looks more complicated.

Pauling was wrong in important ways, but he was not entirely wrong. Modern research is giving vitamin C a second look in cancer, and it turns out that under certain conditions it can behave less like a gentle vitamin and more like a drug. Pauling's vitamin C story began in the 1970s, when he teamed up with the Scottish doctor Ewan Cameron and gave patients with advanced, incurable cancer very large amounts of vitamin C – first as a drip into a vein, then as tablets.

Compared with similar patients who did not get vitamin C, they reported that the vitamin‑treated group lived longer and felt better. For some, they suggested, survival could be several times longer. Two large trials, run by the Mayo Clinic, a leading non-profit medical center in the US, then put this to the test.

The results were clear: there was no benefit. Patients who took vitamin C pills lived no longer than those who didn't. For most oncologists, that was the end of the matter. Vitamin C was filed away with other "alternative" remedies, and Pauling's late-career crusade was widely seen as a sad mistake.

What neither trial's critics nor defenders noticed at the time: Pauling and Cameron had started with vitamin C into a vein; the Mayo Clinic trials used tablets only. That matters because the gut can only absorb so much vitamin C. Once you reach a modest daily dose, the body simply stops taking in much more. Swallow as many tablets as you like, and the level of vitamin C in your blood levels off.

By contrast, a drip into a vein can raise blood levels to tens or even hundreds of times higher than tablets ever could. At those extreme levels, vitamin C starts to behave differently inside the body. At everyday levels, vitamin C acts as an antioxidant it mops up harmful molecules and protects our cells.

At very high levels, especially around tumors, it can flip roles. In laboratory studies, high-dose vitamin C helps generate hydrogen peroxide, a reactive substance that can damage cells. Cancer cells seem especially vulnerable because they are already under stress.

They grow rapidly, often in areas with poor blood supply, and produce lots of reactive molecules of their own. Their internal "cleanup" systems are stretched thin. Add a sudden pulse of hydrogen peroxide and many cancer cells tip over the edge their DNA and energy machinery are damaged and they die.

Normal cells, which are under less strain and have better defenses, are more likely to survive. In this way, very high doses of vitamin C behave less like a daily supplement and more like a weak, selective chemotherapy drug. Crucially, the doses needed for this effect cannot be reached with tablets.

In people, the evidence is still early and mixed. Small trials have given high-dose vitamin C through a vein to patients with hard-to-treat cancers such as ovarian, pancreatic or brain tumors.